The rearrangement of a ketoxime to the corresponding amide was discovered in 1886 by E. Beckmann and is known as the Beckmann rearrangement. The rearrangement is brought about by acids, including Lewis acids. The more common rearranging agents are concentrated sulfuric acid, phosphorus pentachloride in ether, and Beckmann’s mixture, hydrogen chloride in a mixture of acetic acid and acetic anhydride. The Beckmann rearrangement is used frequently to determine the structure of ketones, by identification of the acid and amine obtained by hydrolysis of the amide formed by the rearrangement. There is no uniform convention for the designation of the stereochemistry of oximes in the literature. The conventions used in this chapter for the configurations of ketoximes and aldoximes are explained.